The present invention relates to a semiconductor memory device for storing information by using change in the magnetic resistance occurring attributable to magnetic spins.
These days semiconductor memories have been widely used in a main memory for a large-scaled computer, a personal computer, a home electric product, a portable telephone and the like. The semiconductor memories are, on the market, classified into a volatile DRAM (Dynamic RAM), an SRAM (Static RAM), a non-volatile MROM (Mask ROM), a Flash EEPROM (Electrically Erasable Programmable ROM) and the like. In particular, the DRAM, which is a volatile memory, has advantages of low cost (because the area is 1/4 of the SRAM) and high speed operation (as compared with the EEPROM). Therefore, the DRAM has the largest market share.
The EEPROM, which is a programmable and non-volatile memory, has a problem of a poor number of permitted writing and read operations (too small number of W/E operations) of about 10.sup.6 times, a time of about a microsecond being required to write data and a necessity of applying high voltage (12 V to 22 V) to write data. Therefore, the EEPROM cannot get a large share as compared with the DRAM.
On the other hand, the FRAM which is a non-volatile semiconductor memory using a ferroelectric capacitor and suggested in 1980 has advantages of a large number of permitted writing and read operations of 10.sup.12 times, short time being required to write/read data similar to that required for the DRAM and a low voltage level of 3 V to 5 V being required to be applied. Therefore, the FRAM has been energetically researched and developed by a multiplicity of manufacturers. However, when the number of writing/read operations is 10.sup.12 times, (100 ns.times.10.sup.12 times)/(60.times.60.times.24 seconds)=1.15 days at 100 ns cycle time. Therefore, if the number of writing/read operations is smaller than 10.sup.15 times, a continuous operation for 10 years or longer is not permitted. Thus, the FRAM cannot be used as a main memory like the DRAM.
On the other hand, a non-volatile semiconductor memory using the magnetoresistance effect, such as the GMR (Giant Magneto Resistance) has been researched and developed (for example, J. L. Brown et al, IEEE Trans. of Components Packaging, and Manufacturing Technology-PART A, Vol. 17, No. 3, September, 1994). The GMR memory has advantages of nondestructive readout, high speed operation and high radioactive ray pressure resistance. Moreover, the GMR memory exhibits a large number of writing/read operations of 10.sup.15 times or more. Therefore, the GMR memory has a possibility that the GMR memory will be used in place of the DRAM market, all of semiconductor memories and hard disk (HD).
FIG. 1A is a plan view showing a conventional GMR memory, and FIG. 1B is a cross sectional view taken along line 1B--1B shown in FIG. 1A. As shown in FIGS. 1A and 1B, a GMR film 1 is, in series, connected to bit-lines 2 and 3, while a word line 4 is formed above the GMR film 1 to intersect the bitlines 2 and 3. The GMR film 1 is in the form of a artificial metal lattice, nanogranular alloy or an exchange connection GMR film formed by a sandwich layer composed of thin ferromagnetic layer 11, a non-magnetic conductive layer 12 and a ferromagnetic layer 13 as shown in FIG. 2A. Moreover, a tunnel GMR, a GMR made of an oxide magnetic material and CMR (Colossal MR) have been suggested.
The operation of the GMR memory will now be described in which the exchange connection GMR film will be described. The layers have thicknesses such that the ferromagnetic layers 11 and 13 have thickness of 3.0 nm and the non-magnetic conductive layer 12 has a thickness of 2.0 nm. The thicknesses of the foregoing layers are smaller than the mean free path of electrons. The spin of the ferromagnetic layers 11 and 13 on the two sides of the non-magnetic conductive layer 12 is made to be opposite directions to each other in a state of a zero magnetic field attributable to the mutual exchange effect. The direction of the spin is changed into a direction of the synthesized magnetic field of the magnetic fields (H) generated by the electric current allowed to flow in the word line shown in FIG. 2B and the bitline shown in FIG. 2C. When the directions of the spins on the two sides are opposite, the electric resistance is high, while the electric resistance is low when the directions of the spins on the two sides are the same.
That is, the resistance is determined by only the relative directions of the spins on the two sides, that is, the resistance does not depend upon the absolute directions of the spins on the two sides (isotropy). The GMR memory uses the difference in the resistance to read written information. That is, the difference in the potential generated when an electric current is supplied to the bitline is amplified by a sense amplifier so that information "0"and "1" is read.
FIG. 2B shows a direction of a magnetic field when an electric current has been supplied to the bitline. A circular mark having a dot therein indicates a case where the electric current is allowed to flow on this side, while a circular mark having a mark x therein indicates a case where the electric current is allowed to flow in the deep portion. The Ampere's corkscrew rule causes the electric current in the word line to generate a magnetic field in a direction of the bitline. Thus, both of the ferromagnetic layers generate magnetic fields in the same direction. The electric current in the bitline generates a magnetic field in the direction of the word line. Thus, magnetic fields (hereinafter called as "rotating magnetic fields") in directions opposite to each other are generated with respect to the ferromagnetic layers on the two sides.
A variety of structures of the GMR memory cell have been suggested as shown in FIGS. 3A to 3C.
FIG. 3A shows an exchange connection GMR film in the form of a stacked structure composed of, in the ascending order, a ferromagnetic layer (mainly composed of any one of Co, NiFe, CoFe or NiFeCo), a non-magnetic conductive layer (mainly composed of any one of Co, Ag or Au) and a ferromagnetic layer (mainly composed of any one of Co, NiFe, CoFe or NiFeCo). One of storage methods adapted to the GMR film is arranged such that spins in opposite directions are provided when the magnetic field is low and spins in the same direction are provided when the magnetic field is higher than the saturated magnetic field so that "1" and "0" are stored. Another method is arranged such that spins in opposite directions are provided in the direction of the word line so as to be "0" data item. Moreover, a large electric current is allowed to flow in the word line to direct the both spins into the direction of the bitline and an electric current is allowed to flow in such a manner that a rotating magnetic field is generated in a direction opposite to the spinning direction in the opposite direction so that the spins of the upper and lower ferromagnetic layers in the opposite directions and opposite absolute directions are inverted so as to stored as "1" data item. The spin cannot be rotated by only the rotating magnetic field. When the synthesized magnetic field with the magnetic field generated by the electric current in the word line exceeds energy required for the inversion, the spins can be inverted.
A method of reading the GMR memory cell is arranged in such a manner that an electric current smaller than that for use in the writing operation is allowed to flow into the opposite direction to the direction of the word line to direct the both spins into the direction of the same bitlines. Then, an electric current is allowed to flow in the bitline in a direction in which the rotating magnetic field, which is the same when the "1" data item has been written, is generated. Since the direction of the rotating magnetic field is the same as the direction of the spin when "1" data item is read, the spins are directed in the direction of the word line in the opposite directions regardless of the electric current in the word line. As a result, the resistance of the bitline is raised. If the "0" data item is read, the direction of the spin and the direction of the rotating magnetic field are different from each other. Therefore, force for causing the direction of both spins attributable to the electric current in the word line to direct the direction of the same bit is enlarged (no inversion takes place because the electric current in the word line is small). As a result, the resistance of the bitline is lowered.
FIGS. 3B and 3C shows a non-connection (spin-bulb) film in which the upper and lower magnetic layers of the conductive layer are independently operated. Referring to FIG. 3B, a stacked structure is formed in which a soft magnetic layer (NiFe(Co)), a non-magnetic conductive layer (Cu) and a (semi) hard magnetic layer (CoPt) are formed in this sequential order when viewed from the lower position. The magnetic field in which the spinning direction in the (semi) hard magnetic layer is inverted is high, while that in which the spinning direction in the soft magnetic layer is inverted is low. Therefore, when a large word line electric current is allowed to flow toward this side when viewed in FIG. 3B, the (semi) hard magnetic layer store the "0" data item. When a large word line electric current is allowed to flow, the (semi) hard magnetic layer stores "1" data item.
When data is read, for example, "0" data item is read, left-hand spin is generated in the soft magnetic layer so that the soft magnetic layer has the spin opposite to that in the hard magnetic layer. Thus, the resistance of the soft magnetic layer increases. When "1" data item is read, the spin in the soft magnetic layer is the same as that in the hard magnetic layer. Thus, the resistance of the soft magnetic layer decreases. The difference in the resistance value is read as recorded information. The intensity of the magnetic field may be the synthesized magnetic field of the word line and the bitline or a second word line is provided in a direction perpendicular to the word line and a cell in a portion in which the selected word line and the second word line intersect may be generated by the synthesized magnetic field.
FIG. 3C shows a stacked structure composed of, in the ascending order, a soft magnetic layer (NiFe(Co)), a non-magnetic thin layer (Cu), a soft magnetic layer (NiFe(Co)) and an anti-ferromagnetic layer (FeMn). The anti-magnetic layer causes the soft magnetic layer above the conductive layer is connected strongly attributable to exchange connection and thus the spin is fixed. The spin in only the soft magnetic layer below the conductive layer is inverted attributable to the magnetic field so that data is stored.
However, a GMR memory of the foregoing type has not been put into practical use because of the following critical problems.
FIG. 4 shows a circuit diagram equivalent to the conventional GMR memory. Referring to FIG. 4, a memory cell is indicated by a symbol representing resistance provided with a diagonal line. Since the resistance of the bitline is changed by the magnetoresistance effect, the foregoing symbol is employed in this specification. Referring to FIG. 4, the word line is omitted from illustration. As the memory cell, the cell shown in FIGS. 3A to 3C may be employed. An assumption is performed that the resistance of a bitline in a case where spins in the upper and lower magnetic layers of one cell is opposite is R and the resistance in a case where the directions of the spins are the same is (R-.DELTA.R). FIG. 4 shows a structure in which a plurality of cells are connected in series, one end (Vs) is grounded and another end is connected to a sense amplifier circuit and a constant-current generating circuit through a block select transistor (Q1).
When a read operation is performed such that, for example, "1" data item is read, the resistance of the selected and the other non-selected cells is R. When "0" data item is read, the resistance of the selected cell is (R-.DELTA.R), and the other non-selected cell is R. Assuming that the electric current which flows from the constant current generating circuit to Vint and Vs is I, half of input potential (Vint) to the sense amplifier in the case of the "1" data item and the "0" data item, that is, the amount (Vout) of the signal from the cell is theoretically .DELTA.R.times.I. However, the present GMR has the resistance change rate .DELTA.R/R is a low value of 5% to 30%. Also Vout is a low value of 5 mV to 10 mV. Since the read signal is small as described above, the conventional GMR cell has the following problem.
The electric current I, which flows in the bitline, and the ON resistance (r) of the block select transistor cause IR drop to be generated in the block select transistor. For example, when the number of series cells=16, R=100 .OMEGA., .DELTA.R=10 .OMEGA., r=625 .OMEGA.and Vint=2 V, I=2 V/(100.times.16+625) .OMEGA.=0.89 mA and output Vout=.+-.4.5 mV. On the other hand, potential of I.times.r=0.55 V is applied between the source and the drain of the block select transistor. When the dispersion in the electric current in the transistor is .+-.10%, the output potential is changed by a large amount of .+-.55 mv. Also the noise/signal ratio is undesirably made to be 1000%. When the IR drop of the wiring resistance r' between the cell block and the constant current generating circuit is added, a practical operation as a large capacity memory cannot be performed although operation of one cell block is permitted.
On the other hand, a GMR memory of a type, such as Brown, is arranged to read cell data two times to cancel noise. The foregoing method is arranged such that, for example, the sense amplifier side reads both Vout in which the resistance of the selected cell is R as it is and Vout in which the resistance of the selected cell is (R-.DELTA.R) so as to obtain the difference as an output signal. However, the foregoing double reading method, having a problem in that the sensing operation is made to be excessively slow, encounters a critical problem when the power supply is changed. That is, if the value of Vint is changed by 100 mV between the first read operation and the second read operation, a malfunction occurs.
Although the present GMR film is able to obtain a magnetoresistance change rate (about 100% at room temperature is enabled) by a large magnetic field by supplying a large electric current of 100 mA to several A, electric current consumption cannot be reduced. Since there is the difference between the distance from the word line to the GMR film and that from the bitline to the GMR film, the magnetic field generated by the word line current is undesirably weakened than the magnetic field generated by the bitline. Therefore, a large electric current is required for the word line. If a material having a high resistance change ratio (high MR ratio) capable of excellent sensing sensitivity required to manufacture a reliable LSI is used, a large word line current must be allowed to flow in the chip. If a plurality of word lines are selected, there arises a problem in that a larger electric current consumption is required for the practical use. The foregoing problem can be explained by the relationship shown in FIG. 5 between the magnetic field Oe! and MR change rate (%). The present GMR film permits either a film having a high change ratio and requiring a large magnetic field (indicated with line A) or a film having a low change ratio and requiring a low magnetic field (indicated with line B). Thus, an ideal film as indicated with line Cell requiring a low magnetic field and capable of realizing a high MR ratio cannot be obtained.
Moreover, the conventional technique involves an influence of the magnetic field on memory cells of non-selected cell adjacent to a selected word line when an electric current is supplied to the selected word line to generate the magnetic field. Thus, data on the adjacent cell is broken when data is written. When data is read, change in the resistance of the selected cell is weakened in a case where inverse data has been written on the adjacent cell with respect to the selected cell. The foregoing problem becomes further critical when the structure is formed more precisely.
As described above, the conventional semiconductor memory device, such as the GMR memory, has advantages of nondestructive reading, high speed operation and excellent resistance against high radioactive ray pressure, permitted number of reading/writing operations of 10.sup.15 times or more and continuous operation for ten years. Thus, the semiconductor memory device of the foregoing types is able to get the DRAM market and replace all of the semiconductor memories and hard disks (HD). However, the dispersion of the IR drop and IR drop of the wiring system in the transistor portion, such as the block select transistor, occurs excessively with respect to a small amount of read signal. Thus, the operation as a large capacity memory cannot easily be performed. The method arranged to read data two times has a problem of a slow operation and encounters a critical problem if the power supply voltage is changed.
What is worse, if a magnetic layer required to manufacture a reliable LSI, having an excellent sensing sensitivity and having a high resistance change ratio (a high MR ratio) is employed, a large word line current must be supplied in the chip. If a plurality of word lines are selected, there arises a critical problem in that an excessively larger electric current consumption is required for the practical use. If the structure is formed further finely, there arises a problem in that the leaked magnetic field breaks data on the adjacent cell.